This invention generally concerns optical splice joints, and is specifically concerned with a low loss splice joint and process for joining a microstructured optical fiber with a doped silica optical fiber.
Microstructured optical fibers are optical waveguide fibers typically formed from silica which contain a periodic array of holes on the scale of the optical wavelengths that propagate along the axis of the fiber. The holes are symmetrically arranged within the fiber to form a light guiding microstructure or core. The resulting microstructure provides an array of air-glass interfaces that guides light along the optical axis of the fiber by total internal reflection. While such fibers are more difficult to fabricate than conventional optical fibers (some are formed by stacking, fusing and drawing a bundle of silica capillary tubes), they have the advantage of being able to transmit a broad range of different optical wavelengths in a single mode along the length of the fiber, thereby minimizing intermodal dispersion-type noise in the transmitted optical signals.
One of the obstacles blocking the practical use of such microstructured fibers is the efficient coupling of light into and out of this type of waveguide. Such coupling is usually implemented by splices that optically and mechanically interconnect a pair of optical fibers. Such splices are typically created by a fusion splicing process wherein an electrical arc is used to fuse together the ends of optical fibers being joined.
Unfortunately, when fusion splicing is used to join a microstructure optical fiber with a conventional optical fiber, the resulting joint causes high losses of 1.5 dB or more in the combined fiber. Such losses are far higher than the losses which occur in a splice joint between two conventional optical fibers, which typically are only about 0.02 dB, which corresponds to about a 0.5% signal loss. To put the loss problem in even greater perspective, the minimum 1.5 dB loss associated with a single microstructured optical fiber splice corresponds to at least a 25% loss in signal.
Clearly, there is a need for a splicing process capable of reliably and consistently creating a splice joint between a microstructured optical fiber and a conventional optical fiber without the excessive losses associated with the prior art. Ideally, the resulting splice would not only have losses considerably less than the 1.5 dB associated with the prior art, but would also have a high degree of mechanical strength for resisting breakage or damage to the fiber when it is installed or modified within an optical network. Finally, it would be desirable if such a low-loss, high strength splice could be created quickly, easily, and inexpensively without the need for specially designed and manufactured fusion splicing machinery.
The invention is an optical splice joint and method that fulfills all of the aforementioned needs. The splice joint of the invention generally comprises an end portion of a microstructured optical fiber having a light guiding microstructure and a jacket circumscribing the microstructure, an end portion of an optical fiber, and a fused portion joining the end portions of the fibers wherein the optical losses associated with the fused portion is less than 1 dB and preferably less than 0.30 dB. The inventive splice and process stem from the observation by the inventors that the high losses associated with prior art microstructured optical fiber splices were caused by the substantial collapse of the holes in the microstructure during the fusion steps of the splicing process. By contrast, the holes in the microstructure in a splice of the invention are only partially collapsed less than about 50%, and more preferably to only about 35%. This substantial reduction in hole collapse results in far smaller losses than those associated with the prior art.
The diameter of the mode fields of both the microstructured optical fiber and optical fiber are preferably substantially equal, and the fused portion forming the splice joint has a tensile strength of at least 30 Kpsi and more preferably a tensile strength equal to or greater than 50 Kpsi.
To prevent collapse of the microstructure during the fusing steps of the splicing process, the radial width of the jacket of the microstructured optical fiber is at least 1.6 times the radial width of the microstructure and more preferably about twice such radial width. The optical fiber is preferably a doped silica fiber, and more preferably a 2% high delta fiber, as the mode field diameter of such fibers is about the same size as the mode field diameter of a microstructured fiber (i.e., about 6 xcexcm).
In the process of the invention, an end portion of a microstructured fiber and an optical fiber are first aligned in opposing relationship in a fusion splicer, with the end of the microstructured fiber offset along the axis of the fibers between about 75 and 120 xcexcm from the center of the arc produced by the splicer such that the regular fiber is an extra 75 to 120 xcexcm over the arc. Such offsetting prevents the microstructure in the fiber end from being exposed to excessive heat. An arc is then generated from the electrically operated fusion splicer with a current of between 8 and 12 mA for a time period of between about 0.2 and 0.4 seconds, and the opposing end portions are then moved into contact. After about 0.3 seconds, the contacting end portions are then exposed to the arc for an additional time period of between about 0.3 and 0.7 seconds. The relatively thick outer jacket of the microstructured optical fiber, in combination with the offset positioning of the microstructured optical fiber end and the low current two-stage heating process prevents substantial hole collapse in the microstructure, and results in a splice that is characterized by a loss of less than 0.65 dB and more typically about 0.20 dB along with a high tensile strength.